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WebM does take longer to encode than h264 at the same quality level, but that is its only penalty. If one chooses encoding profiles to yield the same quality level, the WebM codec actually ends up with slightly lower bandwidth (iow a slightly smaller filesize) than H264.

Have a WebM/H264 comparison to verify that?

Here's a fairly thorough one that puts x264 above WebM's standard encoder consistently, both in terms of quality and encoding speed (encoding speed seems to be about 3x faster for x264):